Molecular Mechanisms of Dissolved Organic Matter in Controlling Cr(III) Colloidal Deposition and Reversibility

Transport of Cr(III) colloids in the subsurface poses environmental risks because they can be reoxidized to carcinogenic Cr(VI). Although dissolved organic matter (DOM) is known to determine the fate of colloidal Cr(III) via organic-mineral interactions, the molecular mechanisms controlling its mobility and deposition remain unclear. This study investigated changes in the deposition kinetics and characteristics of Cr(III) colloids in response to DOM composition and concentration. Quartz crystal microbalance with dissipation showed a nonmonotonic deposition pattern of Cr(III)-DOM colloids across DOM concentrations and types. Specifically, the deposition efficiency decreased to a minimum at a critical DOM concentration, above which it increased significantly with the irreversible fraction. This phenomenon was driven by dynamic interactions between silica and the multilayer corona of Cr(III) colloids, arising from the self-assembly of aliphatic compounds with condensed polyaromatics or polyphenols. The mechanism was supported by Fourier transform ion cyclotron resonance mass spectrometry, scanning transmission electron microscopy with electron energy loss spectroscopy, and molecular dynamics simulation. A mechanistic dimensionless model based on normalized deposition mass and sorbed aliphatic abundance successfully predicted the deposition of crystalline Cr(III) colloids under soil, sediment, and aquatic DOM. These findings highlight the importance of the DOM molecular composition in assessing heavy metal colloidal transport and Cr contamination risks.